Refractive Index Sensor Based on Micro nano Fiber Structure for Salt Concentration Measurement in Seawater

In this paper, a micro-nano fiber structure sensor for measuring salt concentration of seawater is proposed. The sensor uses a fusion splicer to fuse a 2 cm long multimode fiber (MMF) between two single mode fibers (SMF28). This constitutes a Mach-Zehnder (MZI) interferometer. The beam propagation method is used to simulate and analyze the microfibers with different waist cone diameters. The microfibers with different stretching lengths are also obtained using the fused tensile cone technique. Finally, the appropriate stretching length was determined to be 25739 μm, The transmission spectra were simulated and analyzed for the waist cone diameter of 25 μ m using the correspondence between salt concentration and refractive index. We used a salt solution with a concentration of 0 to 12 g/L as the analyzing test solution. The transmission spectra of broadband light source were measured after passing through this sensor, and the characteristic valley wavelength method was selected for demodulation. Both theory and experiment showed that the shift of the characteristic valley wavelength showed a good linear relationship with the salt concentration. The sensitivity of this sensor was 340.27 pm/gL.: 1, respectively. The salt concentration sensor in this paper has the advantages of good mechanical strength, simple structure, good repeatability and high sensitivity. Therefore, it can be applied to the measurement of seawater salt concentration.